Coupler systems for modern railroad cars typically include a draft gear assembly for cushioning and absorbing impact forces placed on the system during railcar operations. A conventional draft gear assembly includes an axially elongated housing having a friction clutch or other form of frictional restraint device arranged at one end thereof. A conventional draft gear assembly further includes an elastomeric spring package operably coupled to the friction clutch to absorb, dissipate and return energy imparted thereto during railcar operations.
A conventional elastomeric spring package used in a draft gear assembly includes a series of elastomeric springs arranged in axially stacked relation relative to each other. Each elastomeric spring includes an elastomeric pad having metal plates joined or bonded to opposite ends thereof. Testing has shown, the overall impact absorbing capabilities of each elastomeric spring are affected not only by the elastomeric spring pad design, but by the surface contact and the bonding of the metal plates to the spring pad.
One of the more useful elastomers for forming such railcar elastomeric springs is a thermoplastic polyester elastomer of the type sold by E. I du Pont de Nemoures & Co. under the trademark HYTREL®. Of course, similar elastomers may be produced and sold by other companies. In actual practice, HYTREL® 5550, 5555, 5556HS and 4056 elastomer composites have been used to form elastomeric springs for railcar draft gears. The first two numbers of those elastomeric composites signify the durometer hardness on the Shore D scale.
Generally, HYTREL® has inherent physical properties making it unsuitable for use as a compression spring. Applicants' Assignee, however, discovered a process by which the thermoplastic polyester material used to form the spring pad can be treated for rendering the elastomer suitable as a compression spring. Generally, that treatment, to convert the elastomer into a compression spring comprises the application of a compressive force to an elastomeric preform thereby compressing the preform in an axial direction to an extent greater than 30% of the initial axial length of the preform, measured in the direction of the applied pressure, and thereafter maintaining the preform under compression at a predetermined height for a predetermined dwell period, and then releasing the axial compression on the preform.
Research by Applicants' Assignee has revealed the provision of a central core or opening in the preform, before the application of the axial compressive force to the preform, has substantial beneficial affects on the resultant compression spring. The use of the hollow compression elastomeric spring is enhanced by changing the spring characteristics and thereby enlarging the scope of the applications where such a spring can be utilized. Moreover, providing a central core or opening in the compression spring affords the elastomeric spring with other advantages. For example, the provision of a core opening extending axially through the preform before the application of a compressive force thereto has been found not to cause the sidewalls of the spring to collapse as may be expected. Rather, the sidewalls of the spring and the core opening expand radially outward in a transverse direction when an axial compressive force is applied to the spring. Suffice it to say, the sidewalls of the spring are generally uniform in thickness and symmetrical about the axial centerline or axis of the spring. Moreover, the central bore or opening allows an axially elongated guide rod to be utilized within the draft gear housing thereby facilitating stacking and alignment of the series of elastomeric compressive springs within the draft gear housing.
Various methods have been proposed for securing the metal plates to the elastomeric pad. One method for securing the metal plates to the elastomeric pad is disclosed in U.S. Pat. No. 4,198,037 to D. G. Anderson. This method involves forming one face of each metal plate with surface incongruities. The surface incongruities on each plate are pressed into the ends of a previously formed preform that has already been transmuted into a compression spring. Another method for securing the metal plates to the elastomeric pad is disclosed in U.S. Pat. No. 5,351,844 to R. A. Carlstedt. This method involves providing a boss with an internal flange defining an aperture on each plate. According to this method, and during the axial compression step utilized to transmute the preform into the compression spring, the aperture in each plate receives a central projection provided at each end of the elastomeric preform.
Either and/or both of the above-listed methods for securing the metal plates to the elastomeric pad work well, especially when the thermoplastic polyester elastomer used to form the spring has a Shore D hardness rating of greater than 50. When the elastomer used to form the elastomeric spring has a durometer in the range of about 40 to 45 on the D hardness scale, however, the method disclosed in the above-mentioned '037 Anderson patent requires either repeated axial compression of the elastomer or a longer dwell period in order to enable the elastomer to form about the surface incongruities in a manner satisfactorily securing the metal plate to the elastomer pad. Even when the compression step is repeated, however, the ability of the elastomeric spring having a durometer in the range of about 40 to 45 on the D hardness scale to bond to the metal plates is wanting and frequently fails. Of course, having to repeat axial compression of the spring to accomplish adhesion between the metal plate and elastomer requires time and, thus, increases manufacturing costs. Moreover, inadvertent separation of the plates from the elastomeric spring pad simply cannot be tolerated. The method disclosed in the above-mentioned '844 Carlstedt patent is not particularly suited for use with compression springs having a central bore or opening extending therethrough.
As mentioned, the elastomeric spring package for the railcar draft gear is arranged within a housing and operably combines with the friction clutch to absorb, dissipate and return energy imparted thereto during railcar operation. During operation of the draft gear, heat generated by the friction clutch is imparted to the those elastomeric springs arranged in proximate relation to the friction clutch. As a result, and especially in those springs utilizing an elastomer having a durometer hardness ranging between 40 and 45 on the D hardness scale, the thermoplastic elastomer of those springs tends to radially expand toward an inner surface of the draft gear housing upon axial compression of the spring package. When the elastomer of the spring rubs or otherwise engages within the inner surface of the railcar housing, performance of the elastomeric spring is adversely affected. In extreme cases, and largely due to continued rubbing of the outer surface of the elastomer against the inner surface of the draft gear housing, one or more of the elastomeric springs can fail resulting in poor draft gear performance.
Thus, there is a continuing need and desire for a railcar elastomeric spring having a hollow elastomeric spring member formed from a thermoplastic polyester elastomer having a durometer hardness ranging between about 40 and about 45 on the Shore D scale and which is securely fastened to a pair of metal plates and is preferably configured to resist radial expansion beyond predetermined limits in response to axial compression of the spring.